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Geoengineering could lead to cooler tropics

January 18, 2011 | Proposals to engineer our climate system and artificially cool the planet have spurred considerable discussion as a possible tactic for warding off the climate effects of global warming. But a new study by NCAR scientists warns that it may not be feasible to prevent the polar regions from rapid warming without cooling the tropics to below present-day levels.

The study, led by NCAR scientist Caspar Ammann, explores whether it would be possible to cool Earth’s global mean surface temperature to below the levels of year 2000 if greenhouse gas emissions continue to rise during the next several decades. Using version 3 of the Community Climate System Model, Ammann and colleagues simulated the effects of two geoengineering proposals: injecting sulfate particles into the stratosphere, which would mimic the impacts of a major volcanic eruption, and placing reflectors between the Earth and Sun, which would mimic the impacts of reduced solar output. Both these approaches aim to lessen the amount of solar energy reaching Earth’s surface.

Results of the study show that a major, hypothetical geoengineering project, launched in the 2020s, could indeed return Earth’s mean surface temperature to the levels of 2000.

To achieve this, however, the tropics would need to be cooled below 2000 levels in order to offset the warming at higher latitudes. This is partly because the poles by the 2020s are projected to have lost so much sea ice, which reflects solar heat, that they would not be very sensitive to the impacts of geoengineering. Other factors predicted to warm the poles include atmospheric circulation changes that strengthen midlatitude westerly winds and increasingly warm ocean waters.

The scientists, who published the study in Journal of Geophysical Research, warn that climates in different regions would respond to geoengineering in very diverse ways, much as they respond differently to naturally occurring volcanic eruptions and fluctuations in solar output. “The changes in the Arctic may be so profound in the next couple decades that trying to ‘fix’ the climate there might turn out to be futile,” Ammann says.